The present invention relates to a constant pressure spring particularly adapted for ribbon cartridges of the type commonly used in typewriters and printers. More particularly, the present invention relates to a ribbon cartridge having a V-shaped spring including two arms, a bridge and a pair of coil springs, for exerting a substantially constant and limited force and load on one of the arms even though such arm is moved through an angle of up to 90.degree..
Generally, force applied by a coil spring having two arms is increased when the angle between the two arms of the spring is decreased by moving one of the arms. As the angle between the arms varies due to movement of one arm, it is very difficult to exert a constant force and load on an element engaged or carried by the spring.
Various types of coil spring systems having only one coil spring and two arms are well known in the art as shown in U.S. Pat. Nos. 4,986,678; 4,990,008; and 4,998,833. However, such prior art coil spring systems suffer from difficulty in exerting a constant force and load when the angle between the two arms changes, as, for example, when used ink ribbon winds on a take-up spool from a feed spool of a ribbon cartridge and the effective diameter of the take-up spool holding the used ink ribbon increases. Therefore, such spring systems require additional installations for exerting a constant force and load.
In order to avoid the above problems, U.S. Pat. No. 4,367,052 to Steger discloses a flat spring including a length of resilient material which has terminal end portions and a central body portion. However, the spring of this patent is complicated in structure and is difficult to use in typewriter cartridges.